The plate heat exchangers generally include a stack of metal plates having separated passageways defined therein through which heat exchange fluids flow to exchange heat therebetween. The plate heat exchangers have a large surface area per volume and can be made compact. Because they can be made with a lesser amount of material, they gradually surpass tube and shell heat exchangers in use. In ordinary plate heat exchangers, outer peripheral portions of the plates or header holes are sealed with gaskets, and the plates are mechanically fixed. Although they can be taken apart and cleaned, they have the disadvantage of being limited in the range of temperature or pressure of the fluids to be used.
Japanese Laid-Open Patent Publication No. 63-137793 discloses an improved plate heat exchanger that can overcome the above-described problem inherent in the ordinary plate heat exchangers. This heat exchanger includes metal plates piled up one upon the other, in which fluid passageways are formed by punching and each of them is defined within the thickness of a metal plate. This heat exchanger has the same advantages as those of the ordinary plate heat exchangers, and because the metal plates having the fluid passageways are completely secured together, the heat exchanger does not impose a large limitation in the range of temperature or pressure of the fluids to be used.
FIG. 8 depicts such a plate heat exchanger, a portion of which is taken apart for ease of understanding. As shown therein, the plate heat exchanger includes a plurality of passageway plates 81 each having passageways 86 defined therein as penetrations, and includes a plurality of passageway plates 82 each similarly having passageways 87 defined therein as penetrations. All of the plates are piled up alternately with a partition plate 83 interposed between adjacent passageway plates 81, 82. A stack of these plates 81, 82, 83 is sandwiched between a pair of end plates 84, 85.
Each passageway plate 81 has through-holes 92a, 92b defined therein in addition to the passageways 86, while each passageway plate 82 similarly has through-holes 95a, 95b defined therein in addition to the passageways 87. Each partition plate 83 has through-holes 93a, 93b, 94a, 94b defined therein. The end plate 84 has inlet and outlet pipes 88, 89 for a heat exchange fluid A, and inlet and outlet pipes 90, 91 for another heat exchange fluid B, all of which are secured thereto. The passageways 86 in each passageway plate 81 and the passageways 87 in the adjacent passageway plate 82 are separated by a partition plate 83 and cross at right angles.
The heat exchange fluid A enters the heat exchanger through the inlet pipe 88 secured to the end plate 84, passes through the through-holes 94a, 95a, and enters the passageways 86 formed in the passageway plates 81. The heat exchange fluid A that has passed through the passageways 86 is discharged from the heat exchanger via the through-holes 95b, 94b and then via the outlet pipe 89. On the other hand, the heat exchange fluid B enters the heat exchanger through the inlet pipe 90 secured to the end plate 84, passes through the through-holes 92a, 93a, and enters the passageways 87 formed in the passageway plates 82. The heat exchange fluid B that has passed through the passageways 87 is discharged from the heat exchanger via the through-holes 93b, 92b and via the outlet pipe 91. At this moment, the heat exchange fluid A flowing through the passageways 86 exchanges heat, through two partition plates 83 disposed above and below it, with the heat exchange fluid B flowing through the passageways 87.
The conventional plate heat exchanger of the above-described construction has the following drawbacks.
Because the heat exchange fluids A, B form cross- or rectangular-current flows that are in a heat exchange relationship, and because the cross- or rectangular-current flows are inferior in heat transfer efficiency to countercurrent flows, the conventional plate heat exchanger referred to above requires a heat transfer area greater than that required by a heat exchanger of the countercurrent flow type to obtain a predetermined heat transfer capacity, resulting in an increase in size of the heat exchanger. In order to enhance the heat transfer ability on the side of the heat exchange fluid A in the heat exchanger, if the heat transfer area is increased by elongating the passageways 86, it becomes necessary for the passageways 87 adjoining them via the partition plates 83 to be increased in number or in width. In either case, the total sectional area of the passageways 87 increases, and the speed of the heat exchange fluid B decreases, resulting in a reduction in the heat transfer ability of the heat exchange fluid B.
Diffused junction, bonding, brazing or the like is preferably employed to join the plates together in the plate heat exchanger.
In the diffused junction, a stack of plates is pressurized under vacuum and heated to a temperature slightly less than the melting point of the material of the plates. Because the plates are joined together by virtue of diffusion of the material in the vicinity of the mating surfaces of the plates, a considerably large load is required for the application of pressure during joining, thus requiring relatively large pressure equipment. Accordingly, it is difficult to mass-produce the plate heat exchangers at a low cost.
Bonding is generally carried out by first coating the bonding surfaces of the plates with, for example, an epoxy-based bonding agent, and by subsequently conducting heat curing treatment on the plates that have been piled up one upon the other. Because the joining by bonding is poor in pressure resistance or heat resistance of the bonded portions, the use pressure or temperature of the heat exchangers is considerably limited.
On the other hand, brazing is generally carried out by first coating the bonding surfaces of the plates with a solder or brazing material having a melting point lower than that of the plates, and by subsequently heating the plates, which have been piled up one upon the other, to a temperature greater than the melting point of the solder. The melted solder is diffused into the plates to join them.
In view of the manufacturing equipment or pressure resistance of the heat exchangers, brazing is generally employed in joining the plates. However, if the degree of contact between the neighboring plates during brazing is bad, a gap or gaps are created in the brazed portions of the plates, thus causing leakage of the heat exchange fluids. By way of example, passageways or through-holes are formed in the passageway plates or the partition plates by pressing or punching and, hence, burrs are formed on the processed portions of the plates in the direction of pressing or punching. When the plates are piled up, contact of such burrs considerably deteriorates the degree of contact between the neighboring plates, resulting in poor brazing.